Rubio has told Senate leaders that he will vote against the bill unless the child tax credit is made more generous to help lower-income workers who pay payroll taxes and not regular income taxes.

“Sen. Rubio has consistently communicated to the Senate tax negotiators that his vote on final passage would depend on whether the refundability of the Child Tax Credit was increased in a meaningful way,” said a spokesperson for Rubio.

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Rubio told reporters at the Capitol that the current tax credit is insufficient.

"Right now it's only $1,100. It needs to be higher than that," Rubio said.

"I understand that this is a process of give and take, especially when there's only a couple of us fighting for it," he told reporters. "Given all the other changes they've made in the tax code leading into it, I can't in good conscience support it unless we are able to increase the refundable portion of it."

If Rubio votes against the bill, Republicans can only lose one other lawmaker if they hope to pass the final bill that emerges from a House-Senate conference.

Rubio, speaking to reporters at the Capitol, did not give a number for how much the credit must be increased to win his vote, saying it "certainly has to be higher than it is now."